Same reason they use stickers. Differently printed pieces require unique storage areas, increasing the cost of storing them. Plus designers are given a "budget" of new pieces, pieces in new colors, and unique prints in each set they design. Not giving Gamora a unique chest print gave them the chance to do something else in another part of the set.

The problem isn't whether doctor who is popular in the states, it's about who it's popular with; adults love it. Children, not so much. That's why you don't see who merchandise in American toy aisles, you see it in places where adults shop. And if kids aren't buying it, it's not going to be a big seller for Lego, plain and simple.